are widespread within the upper structural levels of the HHCS in the whole
Zanskar area. The abundance of these intrusions increases from north-west
Zanskar towards the south-eastern parts of this area. In the upper Doda,
Suru, Kishtwar and Warwan regions, the occurrence of leucogranites is
limited to disseminated dikes and sills (Honegger, 1983; Herren, 1987;
Kündig, 1988). The claim of Searle and Fryer (1986) that 30-50% of
the granites in this region are of leucogranitic type was thus quite overoptimistic.
From the region of the Haptal Tokpo valley, south of Padum, and towards
the south-east, the leucogranites start to appear as frequent concordant
intrusions with a size that varies between 5 mm and 600 meters (Herren,
1987). The largest amounts of leucogranites in the whole Zanskar area
are undoubtedly occurring in the south-westernmost part of the HHCS, which
is to say in the region covered by the present work.
In this area,
the leucogranite do indeed represent a large part of the HHCS (fig.
6.9) and they occur as a more or less continuous belt of plutons of
variable size from the Gianbul valley to the Kamirup . It is in the latter
valley that the leucogranites can be seen to show on the surface for the
last time, for they have not been mentioned to occur more to the south-east.
It is however very likely that they are still present at an unexposed
level and that they might be quite close to the surface under the high-grade
metamorphic outcrop south of Sarchu.
Zanskar, and especially the Gianbul valley, represents probably one of
the most interesting regions of the Himalaya for the study of the leucogranites
as it is possible to follow these intrusions all the way from their probable
source region up to their summit along this single valley.
in the Gianbul valley form an intrusion complex that can be divided into
four zones from base to top (fig.
zone is the probable production zone of the leucogranitic melts and
is represented by the high-grade migmatites (fig.
6.2) that we have described in chapter 5. The restites in the
migmatitic zone still contain significant quantities of both plagioclase
and alkali feldspar which suggest that the production of melt occurred
through vapour-absent melting of muscovite (Harris et al., 1993).
zone corresponds to the feeder dikes which are rooted into the migmatitic
zone and ascend more or less vertically through the gneisses of the
HHCS over a distance of about one kilometre (fig
6.3). As this zone is exposed in cliffs which are of very difficult
access, we could not get a close view of the relation between the
dikes and the surrounding country-rock gneisses.
zone is formed by an approximately one kilometre thick belt of massive
leucogranitic plutons fed by the underlying dikes. The lower contact
of these plutons is very irregular. One of these massive plutons is
beautifully exposed in the northern ridge of the Gumburanjun mountain
6.4). Even if this outcrop of leucogranites is relatively small
compared to those of the Gianbul valley, we will use this name for
the whole set of leucogranitic bodies of south-east Zanskar as they
are most likely all connected to each other and because this name
was already used in the literature (Gaetani et al. 1985, Ferrara et
al. 1991; Dèzes et al. 1999). Metric to decametric blocks of
country rocks are preserved as xenoliths both at the base and at the
top of the plutons (fig.
rocks have a fine grained equigranular texture and are composed of
a large amount of quartz (60-70%), with biotite (20-30%), albite (10-15%)
and minor K-feldspar (3-5%) and tourmaline (2-3%). The amount of the
rafts decreases gradually towards the centre of the plutons. These
blocks have an angular shape and show a well marked foliation which
is continuous from block to block and parallel with the main foliation
of the country rocks (fig.
6.4). It thus appears that these xenoliths have preserved their
original orientation and were not tilted within the plutons as would
be expected if they were surrounded by large volumes of leucogranite
in a liquid state. This is a strange feature which we tentatively
attribute to the fact that the leucogranitic intrusions do not form
plutons in the usual sense of the term but that they are the result
of a very high concentration of dikes and sills which intruded as
continuous pulses and aggregated as massive bodies of leucogranites
where the individual veins are indistinguishable from one another.
This interpretation is supported by the fact that in the regions outside
of the plutons it is often impossible to establish a chronological
relation between two dikes, as they seem to be perfectly welded together
at their point of junction. Another interesting feature of the xenoliths
is that they seem to be affected by some kind of leaching process.
One can indeed observe that these rocks, which have no particular
reason to represent a different protolith than the rest of the HHCS
rocks, do show a texture and a mineralogical composition that is quite
different from the usual metapelites or orthogneisses, in the sense
that they are almost completely depleted in muscovite and have a fine
grained equigranular texture (~ hornfelsic texture). That these rocks
also initially contained muscovite is testified by the fact that we
could observe, in thin section, a single perfectly rounded muscovite
grain preserved inside a quartz crystal. Thus, it seems more than
likely that these xenoliths reached metamorphic conditions sufficient
for the breakdown of muscovite and that they also represent a source
for leucogranitic melts. The overall aspect of these leucogranitic
intrusion containing angular rafts of country rocks resembles that
of agmatitic migmatites (Mehnert, 1968). The xenoliths should thus
be considered as restites.
zone represents the structurally uppermost levels of the intrusion
complex. Above the roof of the leucogranitic plutons, a gradually
decreasing array of leucogranitic veins intrudes the country rock
gneisses. These veins ascend vertically for a distance of about 100
meters before most of them are reoriented parallel to the Zanskar
Shear Zone (fig.
6.8) and often strongly boudinaged by extensional movements associated
to the shear zone (fig.
6.6) Several dikes do however penetrate the ZSZ without being
affected by extensional deformation (fig.
6.7), which clearly indicates that these dikes must have intruded
the shear zone after ductile movements have ceased. The upper boundary
of the leucogranitic intrusion coincides with the kyanite zone and
they were never seen above the 10-50 meter thick horizon of calcsilicate
rocks, which is nearly always present at this structural level in
the whole studied area.
and pegmatitic dikes are observed towards the top of the intrusion
complex and especially in the uppermost zone. The pegmatites (up to
10 meters in thickness) contain quartz, K-feldspar, plagioclase, muscovite,
tourmaline, garnet, and beryl. The individual size of these minerals
reaches up to 10 cm (yes, even the beryl!). In one sample from the
kyanite zone, a large (12 cm) K-feldspar crystal incorporated pre-existing
centimetre sized kyanite blades. Aplites, in the absence of biotite
and tourmaline are pristine white with red flecks of euhedral garnets.
Many of the pegmatitic dikes seem to be late as they are mostly undeformed
by the ZSZ.
of leucogranites with pegmatites does not imply hydrous (aH2O=1) conditions
during melting. Clemens (1984) estimated that a granitic melt produced
by muscovite breakdown would be close to saturation, having ~ 10 wt.
% dissolved water at 5 kbar (assuming pure OH on the hydroxyl site
in micas). Crystallization of such melts can liberate a large proportion
of volatiles and consequently leads to the formation of migmatites
without external fluids.